Influence of Specific Anions on the Orientational Ordering of

Aug 6, 2012 - In contrast, anions classified as relatively more kosmotropic in nature (fluoride, ... Jonathan K. Whitmer , Abhijeet A. Joshi , Rebecca...
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Influence of Specific Anions on the Orientational Ordering of Thermotropic Liquid Crystals at Aqueous Interfaces Rebecca J. Carlton, C. Derek Ma, Jugal K. Gupta, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: We report that specific anions (of sodium salts) added to aqueous phases at molar concentrations can trigger rapid, orientational ordering transitions in water-immiscible, thermotropic liquid crystals (LCs; e.g., nematic phase of 4′-pentyl-4-cyanobiphenyl, 5CB) contacting the aqueous phases. Anions classified as chaotropic, specifically iodide, perchlorate, and thiocyanate, cause 5CB to undergo continuous, concentration-dependent transitions from planar to homeotropic (perpendicular) orientations at LC−aqueous interfaces within 20 s of addition of the anions. In contrast, anions classified as relatively more kosmotropic in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate, bromide, and chlorate) do not perturb the LC orientation from that observed without added salts (i.e., planar orientation). Surface pressure−area isotherms of Langmuir films of 5CB supported on aqueous salt solutions reveal ion-specific effects ranking in a manner similar to the LC ordering transitions. Specifically, chaotropic salts stabilized monolayers of 5CB to higher surface pressures and areal densities (12.6 mN/m at 27 Å2/molecule for NaClO4) and thus smaller molecular tilt angles (30° from the surface normal for NaClO4) than kosmotropic salts (5.0 mN/m at 38 Å2/molecule with a corresponding tilt angle of 53° for NaCl). These results and others reported herein suggest that anion-specific interactions with 5CB monolayers lead to bulk LC ordering transitions. Support for the proposition that these ion-specific interactions involve the nitrile group was obtained by using a second LC with nitrile groups (E7; ion-specific effects similar to 5CB were observed) and a third LC with fluorine-substituted aromatic groups (TL205; weak dipole and no ion-specific effects were measured). Finally, we also establish that anion-induced orientational transitions in micrometer-thick LC films involve a change in the easy axis of the LC. Overall, these results provide new insights into ionic phenomena occurring at LC−aqueous interfaces, and reveal that the long-range ordering of LC oils can amplify ion-specific interactions at these interfaces into macroscopic ordering transitions.

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INTRODUCTION Ion-specific phenomena in colloidal and macromolecular systems have been reported for over 120 years (the original work of Franz Hofmeister from 1888 is translated into English in ref 1).1 At elevated concentrations, in particular, ion-specific effects are evident in a wide variety of biological and physiochemical phenomena2−6 including measurements of protein stability,1,7−10 micelle formation,10−13 polymer behavior,14−16 and colloidal aggregation.17−20 In early studies, salts were ranked and classified according to their tendency to stabilize or destabilize the native structure of proteins, leading to the naming of some ions as kosmotropes (generally small and strongly hydrated ions) and others as chaotropes (large and weakly hydrated ions).5 Of relevance to the study reported in this paper, ion-specific effects are also evident in measurements of the surface tensions of aqueous salt solutions21−25 and interfacial tensions of oil−water interfaces.26−28 In particular, Jones and Ray reported the surface tensions of aqueous solutions to decrease upon addition of low concentrations of salts, a result that is noteworthy because it indicates a positive surface excess of ions according to the Gibbs adsorption equation.21,22 Although characterization of the effects of salts on oil−water interfaces (free of surfactants and other stabilizing © 2012 American Chemical Society

agents) is not as complete as that of the surface of water, past reports do describe (i) an increase in the oil−water interfacial tension for aqueous solutions containing kosmotropic anions and (ii) a pronounced decrease in interfacial tension for aqueous solutions containing chaotropic anions (iodide and thiocyanate, up to 0.8 M).26−28 Overall, the above-described measurements of surface and interfacial tensions highlight two key specific ion effects that are not yet fully understood: (i) the origin of the positive surface excess concentration of ions that can form at an interface between water and a second phase with a low dielectric constant (as indicated by a decrease in surface/ interfacial tension), and (ii) the dependence of changes in surface/interfacial tension, in general, on ion-type. Past efforts to provide insight into the above-described interfacial ionic phenomena include additional experiments23−25,29−38 as well as simulations37−42 and theories.43−51 In particular, several theoretical descriptions have been reported in which the potential of an ion within an electrical double layer was modified to account for long-range screened Received: June 14, 2012 Revised: August 4, 2012 Published: August 6, 2012 12796

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design of dynamic and responsive LC interfaces for the sensing of chemical and biological molecules and interactions.52,56−68

image forces, changes in local Born energy of ions near interfaces (with the interfacial region described as possessing a continuously changing dielectric constant), dispersion forces, and other effects.43−51 Here we draw attention to two investigations of particular relevance to the current study, as each predicts the accumulation of ions at oil−water interfaces. Wang and co-workers modeled the interface between two dielectric phases using solution thermodynamics to calculate the solvent composition across the slightly miscible interfacial region. Their evaluation of the chemical potential of the ions included a contribution due to a local Born energy, which established a Galvani potential between the two phases due to the difference in ionic sizes.50 Although this model predicts a positive surface excess of ions based on charge separation at the interface due to differences in Born energy of the ions, the values of the dielectric constants of the oils (εoil = 20 or 40) that were used in the calculations were higher than most experimental values. More recently, dos Santos and Levin reported a model that shows good agreement with experimental measurements of the effects of salts on surface tensions and oil−water interfacial tensions. 45−47 Their approach involved modification of the Poisson−Boltzmann equation at a sharp interface to account for long-range screened image forces, ionic polarizability, position-dependent Born selfenergies, a hydrophobic cavitation energy (i.e., the entropic energy penalty to create a cavity the size of the ion), and dispersion interactions.45−47 Whereas the experimental studies and theories reported above deal with isotropic oils, the investigation reported in this paper moves to consider ionic phenomena at interfaces formed between anisotropic oils (i.e., thermotropic liquid crystals (LCs)) and aqueous phases. We show that the orientational ordering of micrometer-thick films of LCs equilibrated against aqueous solutions of sodium salts can be used to report specific ion effects at LC−water interfaces. The LC−aqueous interface is a particularly interesting type of oil−water interface with which to study interfacial ionic phenomena because past studies have demonstrated that the ordering of LCs at interfaces can report changes in the surface energy of the LC that are on the order of 1−10 μJ/m2.52 Of particular relevance to the study reported in this paper, we recently described the existence of ordering transitions triggered at the LC−aqueous interface caused by ions that form an electrical double layer on the LCside of the interface.53 This effect was seen when the interface was highly charged, which was achieved by equilibrating the LC films against aqueous solutions at high pH (ordering transitions were observed to begin at pH values greater than 9.4, with completely homeotropic orientations induced above pH 12.8).53 In contrast to these past observations, the experimental observations reported in this paper were obtained at values of pH that were below pH 9 (in the aqueous phase). Our observations are, instead, generally consistent with the conclusions of past studies that have used infrared reflection− absorption spectroscopy to characterize ionic phenomena at lipid monolayers54 and vibrational sum frequency spectroscopy to investigate surfactant-laden aqueous interfaces.55 These studies have concluded that chaotropic anions but not kosmotropic anions accumulate (and perturb the ordering of amphiphilic molecules) at these interfaces. We end this introduction by noting that the interactions of LCs with aqueous electrolyte solutions, as investigated in the study reported in this paper, is of relevance to the broad topic of oil− water interfacial phenomena. It is also significant in guiding the

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EXPERIMENTAL SECTION

In a typical experiment, a film of nematic LC with an approximately flat interface was prepared by filling the pores (283 μm × 283 μm) of a 20 μm-thick gold-coated specimen grid supported on a chemically functionalized glass slide (Figure 1a), as detailed elsewhere.69,70

Figure 1. (a−f) Polarized micrographs (crossed polars) and schematic illustrations of nematic films of 5CB in contact with aqueous solutions of (a, b) ultrapure Milli-Q water, (c, d) 2 M NaCl, or (e, f) 2 M NaClO4. Inset in (e) is a conoscopic image confirming homeotropic alignment. In (a−f), the 5CB is supported on an OTS-treated glass surface. Each square is ∼300 μm on a side.

Briefly, glass microscope slides were cleaned according to published procedures69 and coated with octadecyltrichlorosilane (OTS) to anchor the LC in an orientation that was perpendicular (homeotropic) to the LC−glass interface (Figure 1b).57 Immersion of the supported LC-filled grid under ultrapure Milli-Q water (18.2 MΩ·cm) led to the formation of a stable interface between the aqueous phase and LC. In the experiments reported below, LC interfaces thus obtained were equilibrated against aqueous solutions of sodium salts. To quantify the orientation of the LC at the aqueous interface, the optical retardance of each LC film was measured using a Berek U-CTB compensator. The orientation of the LC at the aqueous interface was calculated from the measured retardance using methods described in the Supporting Information (SI). Experimental information regarding the measurement of surface pressure−area isotherms of Langmuir monolayers of LC can also be found in the SI. In some of the experiments described below, a 5CB-filled specimen grid was supported on a gold-coated glass microscope slide on which a monolayer was formed from hexadecanethiol (HSC16). The gold film was formed by physical vapor deposition at an oblique angle of incidence (64° measured from normal), which has been reported previously to lead to a uniform azimuthal orientation of the LC.71 As also reported previously, the HSC16 monolayer causes nematic 5CB to assume an orientation that is parallel to the surface (planar anchoring).72 Additional experimental details are presented in the SI. 12797

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RESULTS AND DISCUSSION In our first experiments, we contacted nematic phases of the nitrile-containing mesogen 5CB (Scheme 1a) with pure water

aqueous solutions of other sodium salts with concentrations of 2 M (or less if limited by solubility) at 25 °C with nematic films of 5CB. We selected the salts according to a previously reported Hofmeister series of anions (Table 1).3,5 We note here

Scheme 1. Molecular structures of the LCs used in this study. (a) 5CB; (b) E7 and (c) TL205

Table 1. Orientations of LCs at Interfaces of Aqueous Salt Solutionsa salts

pH

Co [M]

C [M]

5CB

E7

TL205

NaF Na2SO4 Na2HPO4 NaC2O2H3 NaCl NaNO3 NaBr NaClO3 NaSCN NaClO4 NaI

7.11 6.73 8.86 8.11 6.36 6.08 6.31 7.37 6.62 7.29 6.41

0.98 2 0.54 5.67 6.14 10.83 7.12 9.49 17.14 9 12

0.75 1.5 0.5 2 2 2 2 2 2 2 2

P P P P P P P P T H H

P P P P P P P P T H H

P P P P P P P P P P P

a

Notation: P, planar orientation; H, homeotropic (perpendicular) orientation; T, tilted orientation; Co, maximum solubility at 25°C; C, concentration used. The LCs were supported on OTS-treated glass surfaces (see text for details).

that there exist a number of Hofmeister series in the literature and that the order of the anions is dependent on the phenomenon studied.5,11,54 The salts listed in Table 1 are organized with kosmotropic anions at the top and chaotropic anions at the bottom. Inspection of Table 1 (column 5) reveals the orientation of 5CB observed after 10 min of equilibration of 5CB against each salt solution (the orientations did not change further after an additional hour of equilibration) to correlate with a Hofmeister series.11 Specifically, contact of nematic 5CB with aqueous solutions of kosmotropic anions resulted in planar orientations of the LC, whereas chaotropic anions caused tilted or homeotropic orientations. Here we note that an aqueous 2 M NaSCN solution led to a tilted orientation of 5CB, but that a concentration of 6 M caused homeotropic ordering in 5CB. In contrast to the behavior of NaSCN, kosmotropic salts that caused planar LC orientations at 2 M concentrations still caused planar orientations in LC films at much higher concentrations (see Table S1 in the SI). Figure 2 shows the effect of the concentration of a chaotropic anion (perchlorate) on the orientation of nematic 5CB (Figure

or aqueous solutions containing either 2 M NaCl or 2 M NaClO4 (T = 25 °C; with pH values of 6.4 ± 0.6 and 7.3 ± 0.8, respectively, n ≥ 4). Here we note in advance that the origin of trends in the orientations of LCs described below is not related to variations in pH. We comment also that we avoided the addition of buffering salts to the aqueous phase in order to minimize the number of ion types in our experimental system. Under white light illumination (crossed polars), the 5CB in contact with the pure water exhibited interference colors (Figure 1a) consistent with an LC film that was anchored parallel to the aqueous interface (so-called planar anchoring) and perpendicular to the OTS-treated glass (homeotropic anchoring). The bulk of the LC film is thus splayed and bent to accommodate the hybrid boundary conditions (Figure 1b).57 Upon contact with aqueous 2 M NaCl, the parallel orientation of 5CB at the aqueous interface was unchanged (Figure 1c, d). However, upon equilibration with 2 M NaClO4, we observed the 5CB film to undergo an immediate transition (within 10−20 s) in optical appearance to a dark state (Figure 1e) that corresponded to homeotropic ordering at the aqueous interface (Figure 1f). Conoscopic imaging (inset in Figure 1e) was used to confirm the homeotropic alignment (see SI for details). Confirmation that the bulk phase behavior of the 5CB was not significantly affected by contact with the aqueous solutions containing molar concentrations of salts was obtained through measurements of the clearing temperature of the 5CB, that is, the temperature at which 5CB becomes an isotropic oil. The clearing temperature of pure 5CB was measured to be 35.1 ± 0.1 °C; the clearing temperature of 5CB equilibrated overnight with either pure water, aqueous 2 M NaCl, or aqueous 2 M NaClO4 was 35.0 ± 0.1 °C, 35.0 ± 0.1 °C, and 34.9 ± 0.1 °C, respectively. Overall, the results in Figure 1 indicate that the presence of perchlorate anions in the aqueous phase can trigger an ordering transition in the LC whereas chloride ions do not measurably perturb the orientation of the LC. We explored further the dependence of the ordering of the nematic 5CB on the identity of the anions by contacting

Figure 2. Tilt angles of nematic 5CB (angle from normal at the aqueous interface) after ∼20 min of equilibration against aqueous solutions of either NaCl (squares) or NaClO4 (diamonds) plotted as a function of the salt concentration. 12798

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Figure 3. Time-lapse polarized light micrographs showing an ordering transition in a 5CB film triggered by contact with 2 M NaClO4 (at t = 0 s). Images taken between 1 and 3 s are blurred due to the addition and mixing of the salt solution, which caused variation in the refractive index of the aqueous phase. The inset at 20 s is a conoscopic image confirming homeotropic orientation. Each square is ∼300 μm on a side.

amphiphile-decorated aqueous interfaces have reported that the accumulation of chaotropic anions at lipid- or surfactantdecorated interfaces can perturb the packing of the hydrocarbon tails of the amphiphiles.54,55 To provide further insight into the origins of the LC ordering transitions summarized in Table 1, we characterized the dynamics of the changes in orientation of nematic 5CB. Figure 3 shows a series of time-lapse images that reveal the dynamics of the LC orientation following the addition of 2 M NaClO4 to an aqueous phase. Inspection of the interference colors of the LC in Figure 3 reveals the LC to undergo a continuous tilting transition to a uniformly dark (homeotropic) state within ∼10 s of addition of the salt solution. Here we contrast these dynamics to those observed when LC ordering transitions are caused by formation of electrical double layers at the LC− aqueous interface (as described above). Specifically, in our previous study, the ordering transitions induced by 1 M NaCl solutions at pH 12.8 occurred with characteristic times that ranged from 5 to 60 min.53 In contrast, for the salts and conditions reported in Table 1, we consistently observed all ordering transitions to occur within 20 s. These relatively fast dynamics are consistent with our hypothesis that the ordering transitions are triggered by interactions of the chaotropic anions with an interfacial layer of mesogens in contact with the aqueous phase. That is, the chaotropic anions do not have to diffuse across the interface and into the bulk of the LC film to trigger the ordering transition, as is the case with orientational transitions triggered by the formation of electrical double layers on the LC side of the interface (see our past study for details).53 As a further test of the hypothesis that the ordering transitions reported in Table 1 are triggered by the interactions of chaotropic ions with the interfacial region of the LC, we measured the surface pressure−area isotherms of monolayers of 5CB formed on the surface of aqueous subphases containing a range of salts. Nitrile-containing LCs are known to form stable monolayers at aqueous interfaces.73−77 In particular, past reports have described monolayers of 5CB and 8CB that form over subphases of pure water and inferred that the

2, diamonds), determined via measurement of the optical retardance of the thin LC films. The observation of a continuous, concentration-dependent change in the tilt of the LC was common to all chaotropic anions in Table 1. In contrast, quantification of the tilt angle (measured relative to the surface normal) of the LC in contact with 0−2 M NaCl (Figure 2, squares) revealed no evidence of a change in orientation with increasing salt concentration. Here we emphasize again that the origins of the trends in orientations of the LC evident in Table 1 or Figure 2 are not variation in pH (for the data set reported in Table 1, the pH varied randomly between 6.1 and 8.9). As noted in the Introduction, in our past studies,53 we demonstrated that pH can impact the orientations of LC films through a mechanism that involves formation of an electrical double layer at the LC−aqueous interface (the LC is poled by the internal electric field generated in the diffuse part of the double layer extending into the LC). However, in those studies, pH values in excess of 12.8 were required to achieve sufficient charging of the interface to trigger a homeotropic orientation of the LC. In contrast, inspection of Table 1 reveals that solutions of chaotropic anions can trigger ordering transitions from solutions with pH values below 7 (e.g., pH 6.6 for NaSCN and pH 6.4 for NaI). At the solution conditions shown in Table 1, therefore, we conclude that the influence of the ions on the ordering of the nematic LC is not due to an internal electric field associated with formation of an electrical double layer, a conclusion that is supported further by measurements reported below. Instead, our observations of the effects of specific ions on ordering transitions in the LC correlate directly with past measurements of interfacial tensions of isotropic oil−water systems26−28 that have revealed the formation of a positive surface excess concentration of chaotropic anions at oil−water interfaces. This close correlation with past measurements of oil−water interfacial tensions led us to hypothesize that the LC ordering transitions reported in Table 1 are triggered by interactions associated with a positive surface excess concentration of chaotropic anions in the interfacial region of the LC. In further support for this hypothesis, and as noted above, recent spectroscopic studies of 12799

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mesogens are oriented such that their polar nitrile groups (CN) are pointing toward the water (see schematic in Figure 4a74). These studies have reported an increase in surface

Figure 4. Schematic illustration of a 5CB monolayer (a) supported on an aqueous subphase, which forms a trilayer structure (b) upon compression. Figure adapted from de Mul and Mann.74

pressure at an areal density of 44 ± 3 Å2/molecule (due to coalescence of liquid-phase domains). The subsequent compression of a homogeneous monolayer resulted in measurement of a plateau of the surface pressure, characteristic of the formation of trilayers (Figure 4b), at surface pressures of about 5.0 ± 0.1 mN/m and at an areal density of about 34 ± 3 Å2/molecule (measured at 22−23.5 °C).77−79 In our experiments, the temperature of the subphase was maintained at 25.0 ± 0.1 °C by circulating water at a constant temperature beneath the trough. Figure 5a shows representative surface pressure− area isotherms that we measured for a 5CB monolayer spread on water, or aqueous subphases containing a kosmotropic salt or a chaotropic salt (see Figure S3 in the SI for data obtained with 8CB). Due to the large volume of solution needed to form the subphase, the salt concentration (0.5 M) used in our monolayer studies was lower than that used in the experiments with bulk LCs shown in Figure 1 (typically 2 M). We did, however, perform monolayer isotherm measurements for two salts at a concentration of 2 M (NaCl and NaSCN) to confirm that the difference in salt concentration does not change the conclusions reported below (see the SI for details, Figure S4 and Table S2). Our measurements of the isotherm of a 5CB monolayer over pure water, as shown in Figure 5a, agree closely with previous reports.77−79 Specifically, the surface pressure began to rise (π ≥ 0.25 mN/m) at an areal density of 43 Å2/ molecule, and multilayer formation was indicated by a plateau in surface pressure at 4.3 mN/m at an area of 34 Å2/molecule (numerical averages for isotherm data are summarized in Table 2). A principal effect of addition of the kosmotropic salts (see Figure 5a, and Figure 5b for a comparison of NaF, NaCl, and NaBr) was to cause the initial rise in surface pressure to occur at a larger molecular area (51−55 Å2/molecule, as compared to 43 Å2/molecule measured on pure water). Similar results were found with other kosmotropic salts (not shown). In contrast, we measured the initial rise in surface pressure of monolayers of 5CB over aqueous solutions of chaotropic salts to occur at molecular areas that were comparable to that measured on pure water, with the exception of SCN− (Figure 5c). Additionally, we measured monolayers of 5CB to be stable to much higher surface pressures (10.7 mN/m or more) and, correspondingly,

Figure 5. Surface pressure (π)−area isotherms of 5CB monolayers supported on aqueous subphases containing either (a) water (solid line), 0.5 M NaCl (dotted line), or 0.5 M NaClO4 (dashed line); (b) water or 0.5 M halide salts that span the Hofmeister series listed in Table 1; and (c) water or 0.5 M chaotropic salts.

much smaller molecular areas (24−28 Å2/molecule) in the presence of the chaotropic anions as compared to kosmotropic anions. As shown in Figure 5c, the effect of the anion type on areal density and surface pressure at collapse of the monolayer followed the series NaSCN < NaClO4 < NaI. Here we note also that NaI isotherms reproducibly showed a gradual change in surface pressure with decrease in area at the mono- to multilayer transition. In contrast, all other salts showed an abrupt change in the isotherm. We do not yet fully understand why NaI differed from the other salts in this respect. Finally, the two-dimensional elasticity of the monolayers, calculated as −A(dπ/dA),80,81 reveals that chaotropic anions cause significantly greater monolayer elasticity than do kosmotropic anions (Figure 6, Table 2). The influence of the anions on the minimum molecular area at which monolayers of 5CB are stable is interesting to consider in light of past studies that have estimated the molecular tilt angle of 5CB within monolayers assuming a molecular crosssectional area of approximately 23 Å2/molecule.75,79 From these measurements, we calculate, just prior to multilayer formation, the tilt angle of the 5CB molecules at the surface of the aqueous subphase to be 27° (relative to the interface normal) for NaClO4 and 53° for NaCl (Figure 6, Table 2). While it is well-established that molecular tilt angles in monolayers do not generally predict the bulk tilt angles of LCs,82 we do note the qualitative correlation between the 12800

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Table 2. Properties of the Langmuir Isotherms Shown in Figure 5a aqueous subphase water NaF NaCl NaBr NaSCN NaClO4 NaI

Amono (Å2/molecule) 40 51 55 52 54 50 42

± ± ± ± ± ± ±

2 6 2 5 6 8 6

πc (mN/m)

Ac (Å2/molecule) 32 38 38 36 28 27 24

± ± ± ± ± ± ±

2 4 1 3 4 3 2

4.1 4.6 5.0 5.5 10.3 12.6 17.7

± ± ± ± ± ± ±

0.2 0.1 0.1 0.1 0.4 0.4 1.6

interfacial tilt angle (deg) 44 52 53 49 37 30 13

± ± ± ± ± ± ±

3 6 1 4 6 12 9

elasticity (mN/m) 23 18 20 22 35 51 81

± ± ± ± ± ± ±

2 3 2 4 2 12 25

a With the exception of water, the aqueous subphase contained 0.5 M of the indicated salt. Notation: Amono is the mean molecular area at which the surface pressure rose to 0.25 mN/m. Ac and πc are the mean molecular area and surface pressure at collapse (multilayer formation), respectively. The fifth column is the interfacial tilt angle of 5CB molecules within the monolayer calculated from Ac (see text for details). The sixth column reports the maximum elasticity of the 5CB monolayer on the indicated subphase. Averages and errors are reported from multiple experiments, n ≥ 3.

Figure 6. diamonds, right axis) containing

smaller dipole moment (similar to 1,2-difluorobenzene: μ = 2.6 D).84 Using the procedures described above to characterize the effects of salts on micrometer-thick films of 5CB, we measured the orientations of E7 and TL205 following contact with aqueous solutions of sodium salts (Table 1). Inspection of Table 1 reveals that E7 responds to specific anions in a manner identical to 5CB, with chaotropes inducing tilted or homeotropic ordering (E7 displayed homeotropic anchoring when contacted with 8 M NaSCN). In contrast, TL205 differed from the two nitrile-containing LCs in its response to salts in that a planar orientation of the nematic phase was observed for all salts. The absence of a measurable effect of the chaotropic salts on the ordering of TL205 is consistent with our proposal that the nitrile group of 5CB plays a key role in mediating the effects of specific anions on the LC ordering transitions. The nitrile group could, for example, participate in a dipole (nitrile)induced dipole (chaotropic anion) interaction because the nitrile group of 5CB possesses a relatively large dipole moment and the chaotropic anions are relatively large and polarizable. We make four additional comments regarding the results described above. First, we note that our interpretation of these results is generally consistent with prior studies (see above for details) that have reported the accumulation of chaotropic anions at lipid- or surfactant-decorated interfaces and perturbation of the hydrocarbon tails of the amphiphiles.54,55 Second, the results also appear to fit within the conceptual framework of the theory reported by dos Santos and Levin, to the extent that the theory includes consideration of the role of the polarizability of ions in mediating the accumulation of the ions at fluid interfaces.45−47 The development of a full understanding of the cause of the ordering transitions in LC films that are triggered by chaotropic anions may, however, also require consideration of the dielectric, flexoelectric, and ordoelectric properties of the LCs.85,86 Third, we note that the orientation of the LC is expected to modulate the adsorption of ions since the LC orientation will determine the dielectric environment experienced by an ion as it approaches the interface (and thus influence the Born energy of the ion). In this context, we comment that past reports have described how the orientational ordering of LCs can influence the adsorption of amphiphiles at LC interfaces.87 The influence of the orientational order of the LC on adsorption of ions will be the subject of a future study. Finally, we end these comments by noting that Langmuir monolayer experiments described above could not be performed with TL205 because it did not spread into stable monolayers (presumably due to the relatively weak dipole moment of the mesogen).

Calculated values of the molecular tilt angle (black left axis) and two-dimensional elasticity (gray squares, of monolayers of 5CB supported on aqueous subphases the indicated salts (isotherms shown in Figure 5).

effects of the anions on the tilt angles of 5CB in monolayers at the surface of water and bulk LC orientations, as reported in this paper. Additionally, we note that NaSCN causes a change in the 5CB isotherm that is intermediate between NaClO4 and NaCl (Amono and elasticity similar to the kosmotropic anions and maximum surface pressure resembling the other chaotropic anions). Such intermediate behavior correlates with the observation reported in Table 1 that the effect of NaSCN on the tilt of the micrometer-thick 5CB films is intermediate between that of ClO4− and Cl− (Table 1). While spectroscopic studies are underway to provide additional insight into the intermolecular interactions underlying the influence of chaotropic anions on the surface pressure−area isotherms of 5CB reported in Figure 5, we comment here that formation of a multilayer of 5CB (upon monolayer collapse) involves the transfer of 5CB molecules from an environment in which the nitrile group is available for interaction with ions and water in the aqueous phase to an environment in which the nitrile group is sequestered within a bilayer of 5CB removed from the aqueous phase (Figure 4). This observation hints that the stabilizing influence of the chaotropic anions on the monolayer of 5CB (as determined by πc (pressure at collapse) and elasticity) may be a consequence of an effective attraction between the chaotropic anions and the nitrile group of the 5CB. While speculative, support for the proposition regarding the possible role of the nitrile group in mediating interactions with the anions was obtained by performing experiments with two additional nematic LCs; E7 (a mixture of nitrile-containing mesogens) and TL205 (a mixture containing a fluoro-phenyl functional group) (Scheme 1). We note that E7 has chemical functionality and a dipole moment similar to 5CB (μavg = 4.96 D for 5CB at 25 °C),83 whereas TL205 does not possess a nitrile group and has a 12801

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The final experiment that we report in this paper addresses the influence of the salts on the anisotropic surface energy (γaniso) of the LC. Specifically, we sought to determine if the influence of the specific anions on the orientational ordering of LCs, as reported in Figure 1 and Table 1, was caused by a change in the orientation of the LC corresponding to the lowest interfacial free energy (the so-called easy axis, ϕe) or a change in the depth of the free energy minimum corresponding to the easy axis (so-called anchoring strength, W, as defined by eq 1):86 γaniso =

1 W sin 2(θ − ϕe) 2

(1)

where θ characterizes the orientation of the LC director at the interface. Relevant to this question, as shown in Figure 1b, the initial state of the LC contacting pure water is elastically strained by the hybrid boundary conditions. Under these conditions, a decrease in anchoring strength at the aqueous− LC interface will permit the anchoring at the OTS-treated surface to dictate the orientation of the LC across the entire film (via the effects of the elasticity of the LC; Figure 1f). To probe the origin of the ordering transitions observed in our experiments, we quantified the orientation of 5CB in contact with aqueous solutions of either NaCl, NaSCN, NaI, or NaClO4, when the LC film was supported on an obliquely deposited gold film treated with hexadecanethiol (HSC16; HSC16-treated gold surfaces cause the LC to align parallel to the surface). In this experiment, the oblique deposition of the gold, when combined with the effect of the monolayer formed from HSC16 on the LC, leads to a preferred azimuthal direction (i.e., parallel to the direction of gold deposition),71 which propagates to the aqueous interface since this interface has degenerate planar anchoring (i.e., no preferred azimuthal orientation). Because the initial state of the HSC16-supported LC film that contacts pure water is not strained (Figure 7b), any change in the LC orientation at the aqueous interface caused by addition of salt would necessarily strain the LC (and cannot, therefore, be driven solely by a decrease in W). Polarized light micrographs of the 5CB films used in these measurements revealed that addition of NaCl to the aqueous phase resulted in no measurable change in the ordering (easy axis) of 5CB at the aqueous−LC interface (Figure 7a versus c, with corresponding schematics in Figure 7b and d, respectively). In contrast, addition of NaClO4 resulted in a clearly observed change in the LC ordering (Figure 7a versus e, with corresponding schematics in Figure 7b and f, respectively). Quantitation of the optical retardance of these 5CB films after 5−10 min of contact with the salt solutions allowed us to calculate the tilt angle of the director at the aqueous interface (with respect to the surface normal). From these measurements, we determined that contact of the nematic 5CB with 2 M NaClO4 caused nearly homeotropic ordering of the LC at the aqueous interface (Figure 7e, f), with a tilt angle of 4 ± 3° (measured from the surface normal). In contrast, when contacted with aqueous phases containing 2 M of either NaI, NaSCN, or NaCl, the 5CB films supported on HSC16 exhibited tilt angles of 30 ± 9°, 44 ± 6°, and 90 ± 13°, respectively, at the aqueous interface. From these observations, we conclude that the ordering transition induced by the chaotropic anions in Table 1 is due to a change in the easy axis of the LC, and not solely a decrease in the anchoring energy.

Figure 7. Polarized light micrographs and schematic illustrations of 5CB films supported on a gold-coated glass slide functionalized with HSC16: Nematic 5CB in contact with (a, b) water, (c, d) aqueous 2 M NaCl for 10 min, or (e, f) aqueous 2 M NaClO4 for 10 min. The bright interference colors of 5CB in contact with 2 M chaotropic salts indicates that the easy axis of 5CB at the aqueous interface is influenced by the introduction of chaotropic anions in water.

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CONCLUSIONS The key result reported in this paper is that thermotropic LCs containing nitrile groups undergo anion-specific ordering transitions at interfaces to aqueous salt solutions. Only chaotropic anions, which have been reported previously to exhibit positive surface excess concentrations at isotropic oil− water interfaces,26,27 trigger the ordering transitions in the two nitrile-containing LCs. Kosmotropic anions do not trigger orientational transitions in the three LCs investigated in our study. Inspired by previous spectroscopic studies of monolayers at the surface of water that have revealed chaotropic anions to perturb the packing of the aliphatic tails of amphiphiles,54,55 our results support the hypothesis that chaotropic anions penetrate into the interfacial layer of the LC in contact with the aqueous phase, interact with the relatively large dipole of the nitrile group to perturb the organization of the mesogens at the interface, and thereby change the easy axis of the LC. Support for this proposition is found in three key experiments reported in this paper. First, measurements of pressure−area isotherms of 5CB monolayers supported on chaotropic salt solutions revealed that chaotropic anions stabilize monolayers of 5CB (against collapse into trilayers) at smaller area/molecule values as compared to pure water or kosmotropic salt solutions. This result can be interpreted in terms of an effective attraction between the nitrile-containing mesogens and the aqueous solution of chaotropic anions. Second, ordering transitions induced by specific anions were observed for nitrile-containing LCs (nematic 5CB and E7) but not for TL205. This result is consistent with our proposition, as TL205 possesses a weak dipole moment, and thus interacts weakly with polarizable anions. Third, consistent with the proposal that the interaction of the salt and the LC occurs at the interface of the two phases 12802

dx.doi.org/10.1021/la3024293 | Langmuir 2012, 28, 12796−12805

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and does not require diffusion of the salt into the bulk of the LC, the dynamics of the transitions induced by chaotropic anions are much faster (